Acrylic precursor fibers suitable for preparing carbon or graphite fibers

ABSTRACT

An acrylic fiber useful in the preparation of precursor fibers for the preparation of carbon or graphite fibers contains 93.0-99.4 mol percent acrylonitrile, 0.6-4.0 mol percent of ammonium or amine having a pKb of 5 or less as neutralizing cations for sulfonate and sulfate end groups derived from the initiator and activator and as neutralizing cations for sulfonate groups derived from one or more sulfonic acid containing comonomers and 0-3.0 mol percent of one or more comonomers selected from the group consisting of simple acrylate or methacrylate esters, simply vinyl esters, styrene, vinyl chloride and vinylidene chloride, the fiber containing no more than 0.3 mol percent of cations other than ammonium or amine.

This application is a continuation-in-part of application Ser. No.062,908 filed Aug. 1, 1979, now abandoned.

FIELD OF THE INVENTION

This invention relates to an acrylic fiber suitable for use in a processfor preparing a precursor fiber for production of carbon or graphitefibers.

Conversion of acrylic fibers to carbon or graphite fibers useful asreinforcing materials has been known for many years. In the knownprocess, the acrylic fibers are first heated for one to two hours at200°-300° C. in an oxidizing atmosphere to provide a cyclized precursorfiber which is then heated at 800°-1500° C. in an inert atmosphere toform carbon fibers or to even higher temperatures to form graphitefibers. This process is costly because of the slowness of thecyclization step.

Heating acrylic fibers in an oxidizing atmosphere causes formation of acyclic structure consisting of naphthyridine rings and crosslinking. Thenaphthyridine ring-containing fibers are sufficiently resistant towardmelting so that they can be heated at the high temperatures required toconvert them to carbon or graphite fibers.

The temperature of the cyclization step is limited by the meltingbehavior of the fibers being treated. Use of relatively hightemperatures in order to provide a higher rate of reaction causesdifficulty in that the exothermic cyclization reaction is difficult tocontrol, resulting in loss of fiber properties, fused filaments andnonuniformity in the product. Inclusion of various cross-linkingcomonomers in the acrylic fibers has been suggested as a way to increasethe temperature resistance of the fibers so that the cyclization stepcan be carried out at a higher temperature.

Japanese Patent Application Publication (JPAP) No. 7531/78 suggests forthe preparation of carbon fibers an acrylic fiber containing across-linking comonomer along with another comonomer which is theammonium or amine salt of a sulfonic acid. Carbon fibers from thisprecursor fiber are alleged to be stronger and to provide greaterinterlaminar strength in resin composite structures. JPAP No. 7531/78does not recognize any improvement in the rate of the cyclizationreaction. Cross-linkable copolymers are undesirable in fiber-spinningprocesses in which the polymer solutions are heated as in dry spinningsince premature cross-linking can occur resulting in gelation of thesolution, causing serious economic penalty or even preventingmanufacture.

This invention provides an acrylic fiber suitable for the preparation ofcarbon or graphite fibers which can be heated rapidly to hightemperatures without interfilament fusion or loss of ultimate carbon orgraphite fiber strength. Use of higher temperatures permits completionof the cyclization reaction in a much shorter time, thus making thecyclization process more economical.

The fiber of this invention is an acrylic fiber of an acrylic polymercontaining 93-99.4 mol percent acrylonitrile units, 0.6-4.0 mol percentammonia or amine having a pKb of 5 or less incorporated into the polymeras neutralizing cations for sulfate or sulfonate end groups derived fromthe initiator and activator, if present, and as neutralizing cations forsulfonate groups incorporated into the polymer by copolymerization ofone or more copolymerizable, sulfonate containing comonomers selectedfrom the group consisting of styrene sulfonic acid, allyl sulfonic acid,methallyl sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid andethylene sulfonic acid and 0-3.0 mol percent of units of one or moreneutral comonomers selected from the group consisting of alkyl acrylateshaving 1-4 carbon atoms in the alkyl group, alkyl methacrylates having1-4 carbon atoms in the alkyl group, vinyl acetate, vinyl propionate,styrene, vinyl chloride and vinylidene chloride and no more than 0.3 molpercent neutralizing cations other than ammonia or amine having a pK_(b)of 5 or less. Preferably, the fibers contain no more than 0.1 molpercent of cations other than ammonium or amine. Preferably sulfate andsulfonate end groups, when present, are derived from ammonium persulfateinitiator and ammonium bisulfite activator.

Preferably the precursor fibers contain 0.8 to 2.0 mol percent sulfonicacid containing comonomer. Preferably the sulfonic acid containingcomonomer is 2-acrylamido-2-methylpropanesulfonic acid (AMPS). Mostpreferably no neutral comonomer is present.

The above fibers are useful in a process for preparing precursor fibersfor the preparation of carbon or graphite fibers wherein the fibers ofclaim 1 are heated for 4-20 minutes in one or more stages in air at atemperature at least 10° C. below the stick temperature of the fibersentering that stage to provide fibers having a density of at least 1.36g/cm³. Preferably the acrylic fibers are heated in air at 250°-360° C.Most preferably the process is continued until the precursor fibers havea density of at least 1.40 g/cm³.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plot of fiber stick temperature against time for (1) anacrylic fiber containing 98.8 mol percent acrylonitrile and 1.2 molpercent 2-acrylamido-2-methylpropanesulfonic acid ammonium salt, (curveA), (2) an acrylic fiber containing 95.4 mol percent acrylonitrile, 0.8mol percent itaconic acid and 3.8 mol percent methyl acrylate (curve B)and (3) an acrylic fiber containing 98.8 mol percent acrylonitrile and1.2 mol percent sodium styrenesulfonate (curve C), all samples beingheated in air, fibers (1) and (3) at 280° C. and fiber (2) at 250° C.(highest possible without damaging the fiber).

FIG. 2 is a plot of fiber density against time of heating in air forfiber (1) above heated at 270° C. for 2 minutes, 310° C. for fourminutes and 360° C. for 2 minutes (curve D), for fiber (2) above heatedat 250° C. (curve E), and for fiber (3) above heated at 250° C. (curveF).

FIG. 3 shows differential thermal analysis curves obtained on heatingtwo fiber samples in air or nitrogen at 50° C./min. Fiber (1) above isheated in air (curve G), and in nitrogen (curve H). Fiber (3) above isheated in air (curve I) and in nitrogen (curve J).

FIG. 4 shows a series of differential thermal analysis curves obtainedat 20° C./min. under nitrogen for fibers of a polymer containing 98.8mol percent acrylonitrile and 1.2 mol percent2-acrylamido-2-methylpropane sulfonic acid as the ammonium salt (curveK), the sodium salt (curve L), 0.80 mol percent ammonium salt-0.40 molpercent sodium salt (curve M), and 0.64 mol percent ammonium salt-0.56mol percent sodium salt (curve N).

FIG. 5 shows differential thermal analysis curves obtained at 20° C./minunder nitrogen for fiber (1) above (curve O) and fiber (2) above (curveP). Stick temperatures of the fibers are indicated by arrows below thecurves.

DETAILED DESCRIPTION OF THE INVENTION

The acrylic fibers of this invention are useful in any process for thepreparation of carbon or graphite fibers. They offer substantialadvantages in the speed of cyclization, resulting in a significantreduction in the cost of the overall carbonization/graphitizationprocesses. The largest improvement is realized by use of a temperatureprogram during cyclization in which temperature is increased rapidlywith the provision that it must never be higher than 10° below theincreasing stick temperature of the fiber.

The cyclization reaction is substantially complete when the startingfibers having a density of about 1.18 g./cm.³ have achieved a density ofat least 1.36 g./cm.³ and preferably at least 1.40 g./cm.³. Such fibersare totally insoluble in hot polyacrylonitrile solvents. The cyclizedintermediate fibers having a density of at least 1.36 g./cm.³ may beconverted to carbon or graphite fibers by methods known in the art,e.g., heating the intermediate fibers in an inert gas at 800°-1500° C.or higher for a short period of time. Carbon fibers will have a densityof about 1.70 g./cm.³ and graphite fibers ordinarily have a density inthe range of 1.85-1.95 g./cm.³.

Suitable sulfonic acid containing monomers are styrenesulfonic acid,allylsulfonic acid, methallylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, and ethylenesulfonic acid. The sulfonic acid containingmonomers are preferably polymerized as the ammonium or amine salts butalternatively may be polymerized as the free acid or metallic salt andthen converted to the ammonium or amine salt by ion exchange.

A fiber containing 1 mol percent alkali metal cation as copolymericsulfonate will exchange about 0.8 mol percent of the alkali metal cationwith ammonium ion on soaking 2 hours at room temperature in 5% aqueousammonium sulfate.

Amounts of sulfonic acid ammonium or amine salt containing comonomermuch greater than about 2 mol percent increase the water sensitivity ofthe starting fibers without providing much further increase in catalyticactivity. For this reason, a maximum of 4 mol percent sulfonic acidcontaining monomers is specified.

Suitable amines for forming the amine salts are those amines having apKb of 5 or less such as methyl, ethyl, dimethyl, diethyl, triethyl,ethanol, diethanol amines.

The ammonia or amine groups bound to the acrylic polymer chains viasulfonic acid groups are believed to act as catalysts in the cyclizationreaction permitting a rapid increase in the resistance of the fibers tohigh temperatures. Ammonia or amine salts of sulfonic acid or sulfateend groups, when present, also act as catalysts. In the usual range ofmolecular weights, 0.2-0.35 mol % of ammonium or amine ion can beassociated with end groups.

Acrylic polymers suitable for the preparation of the fibers of thepresent invention may be prepared by conventional free-radicalpolymerization procedures, such as systems employing redox catalysts, insuspension, solution or emulsion systems. Preferably, the polymerizationis carried out in a system containing no metallic cations or at least asystem containing only a low level of metallic cation.

The acrylic fibers may be prepared by conventional solution-spinningprocesses such as dry spinning, wet spinning or dry-jet wet spinning.Dry spinning is preferred.

The acrylic fibers are preferably drawn 2 to 8×. Drawing is preferablycarried out in hot water (e.g. 90° C.) or in steam. The drawn fibers maybe dried by conventional procedures but are preferably dried in a wayproviding precursor fibers having a density of about 1.18 g./cm.³. Ifthe acrylic fibers of the invention are to be prepared by ion exchange,this is preferably carried out on fibers which have not been dried andare still water swollen from the extraction and drawing steps of themanufacturing process.

From FIGS. 1 and 2 it can be seen that fibers useful in the process ofthe present invention are converted to substantially completely cyclizedintermediate fibers having a density of at least 1.40 g./cm.³. much morerapidly than other acrylic fibers.

From FIG. 3 it can be seen that fibers useful in the process of thepresent invention undergo an exothermic reaction (cyclization) both innitrogen and in air at a lower temperature than fibers not useful in thepresent invention and that a smaller exotherm occurs in a non-oxidizingatmosphere (nitrogen).

From FIG. 4 it can be seen that an increasingly larger portion of theexothermic reaction occurs below 300° C., the initial stick temperature,as the amount of ammonium ion increases.

From FIG. 5 it can be seen that a fiber according to the inventionundergoes exothermic reaction below its stick temperature under nitrogenwhile a carbon fiber precursor of the prior art does not. It will benoted that all of the inert-atmosphere DTA curves for the production ofthis invention exhibit an exothermic reaction before the sticktemperature is reached, which is unique for the composition. Thecomparison is best seen in FIG. 4 where a composition having no ammoniumor amine neutralizing cations is compared with others having variouslevels of ammonium-ion content. Evidently, the low temperature reactionoccurring in the fibers of the invention makes possible a fasterconversion without filament fusion because the fiber stick temperaturebegins increasing at a lower temperature.

TESTS Differential Thermal Analysis

A discussion of differential thermal analysis (DTA) appears at p. 263,et seq., of Physical Methods in Macromolecular Chemistry, Carroll, Vol.1, 1969, Marcel Dekker, authored by Feng and Freeman. The methodprovides a measure of the heat absorbed or generated in a sample as afunction of temperature. In the present invention, this method providesa useful comparison of total heat and rate of the exothermic reactionleading to the "cyclized" form of an acrylic fiber which is suitable forrapid carbonization or graphitization at high temperature. The resultsare essentially identical whether polymer or fiber samples are used.

In this analysis, a 5 mg sample is placed in a sample cup which is inturn placed on one loop of the differential thermocouple in a DTA cell(the "Stone" cell, Traco Model SH-15BR2-Ni, is suitable, among others).An empty cup is placed on the other loop.

The temperature is programmed to rise at 20° C./min. or 50° C./min. from100° C. to 400° C.

Density

Density determinations are made in calibrated density gradient tubes asknown in the art. A container such as a standard 250 ml graduatedcylinder contains a fluid prepared to have the desired density gradientand calibrated by the addition of "floats" of selected, known density.The fiber to be tested is knotted, the ends clipped and the knot droppedinto the tube. When it has settled to its equilibrium level, itsposition is read in terms of proximity to calibration floats above andbelow it. Interpolation between these points gives the sample density.

Fiber Stick Temperature

The measurement of fiber stick temperature is an adaptation of the"Procedure for Melting Point Determination", ASTM D-276-62T. Two changeswere made to improve accuracy in the measurement of stick temperature ofa nonmelting fiber such as the fiber of this invention:

Temperature is measured with a reliable surface pyrometer rather than asubmerged thermometer.

A convexly curved surface is used to heat the bare fibers; no coverglasses are used.

In the actual measurement, a strand of fibers is held against the convexsurface for a maximum of 10 seconds. If sticking occurs, the temperatureis dropped about 5° C. and the test repeated; if the filaments do notstick, the temperature is raised about 5° C. and the test repeated. Afresh sample is used for each test. Once sticking occurs, the tests arerepeated at temperatures near this value until successive trials showincidence of sticking within a 2° C. range.

Polymer Preparation

The acrylonitrile polymers suitable for use in making the acrylic fibersof the present invention are preferably made in the conventional aqueoussuspension system as generally taught in the Sampson et al. U.S. Pat.No. 3,308,109 with recipe modifications appropriate to production of thepolymer useful in the present invention. This is a continuous,steady-state redox (e.g., bisulfite-activated/persulfate initiated)polymerization in which all ingredients are metered to an agitated,jacketed vessel, a representative portion of the contents overflowingconstantly. Polymer and unreacted monomers are recovered from theoverflowing slurry. The heat of reaction is removed by water circulatingthrough the jacket.

PREPARATION A

The following ingredients are continuously fed:

    ______________________________________                                                      Parts, by Weight, per Hour                                      ______________________________________                                        Demineralized Water                                                                           75                                                            Acrylonitrile   23.3                                                          AMPS*           1.67    (dissolved in part of the                                                     water feed)                                           SO.sub.2        0.2     (dissolved in acryloni-                                                       trile)                                                (NH.sub.4).sub.2 S.sub.2 O.sub.8                                                              0.07    (dissolved in part of the                                                     water feed)                                           NH.sub.4 HSO.sub.3                                                                            0.2     (dissolved in part of the                                                     water feed)                                           Fe.sup.++       4 ppm   (on feeds, as ferrous                                                         ammonium sulfate)                                     To overflowing slurry:                                                                        in excess of that needed to                                   Sodium-neutralized ethylene                                                                   complex the iron                                              diamine tetraacetic acid                                                      ______________________________________                                         *2-methyl-2-acrylamidopropanesulfonic acid neutralized to pH 2.5 with         NH.sub.4 OH                                                              

The reactor has a working capacity to the continuous overflow of about50 parts, resulting in a residence time of 30 minutes. The temperatureis controlled at 60°±1° C. The pH of the reacting mass is 2.6. Theacrylamidomethylpropanesulfonic acid content of the polymer is 0.7 molpercent. The total ammonium ion content is 1.07 mol percent.

Overall conversion is found to be 78% of a polymer having an intrinsicviscosity of 1.11. The metal ion content is found by analysis to be <10ppm (<0.002 mol % on polymer).

Two additional polymers are made as above, using feeds as follows:

    ______________________________________                                                     Parts, by Weight, per Hour                                                     B          C                                                    ______________________________________                                        Water          80           80                                                Acrylonitrile  18.0         18.8                                              AMPS           2.0          1.2                                               (NH.sub.4).sub.2 S.sub.2 O.sub.8                                                             0.06         0.06                                              NH.sub.4 HSO.sub.3                                                                           0.03         0.06                                              Fe.sup.+       2 ppm        2 ppm                                             pH             3.0          2.8                                               Conversion     80%          --*                                               Intrinsic Viscosity                                                                          2.4          2.8                                               AMPS Content** 1.5 mol %    0.74 mol %                                        Total Ammonium 1.8 mol %    0.92 mol %                                        ______________________________________                                         *Not determined; about equivalent to that of B.                               **By Xray fluorescence to give total sulfur and correction for the end        groups calculated from the intrinsic viscosity                           

It should be noted that the end groups derived from the initiator andactivator are a significant proportion of the total ammonium- oramine-binding capacity. A polymer of lower molecular weight, thusrequires less comonomeric sulfonate.

EXAMPLE I

Polymers are separately dry spun and wash drawn, as known in the art, to1500-filament, 1.5 dpf yarns for this experiment. Polymer D is made bythe prior art procedure, employing K₂ S₂ O₈ as initiator, sodiumbisulfite as activator and sodium styrenesulfonate as the comonomer.Otherwise, the general procedure of the foregoing preparations isfollowed. Polymer E is prepared, as illustrated under the foregoingpreparations. Polymer D contains 1.0 mol % of sodium styrenesulfonate.Polymer E contains 1.0 mol-% AMPS as the ammonium salt.

The yarns are passed continuously at constant length through a tubularfurnace at such a rate as to reach a density of 1.4 g/cc in one pass.The yarn from Polymer D required 96 minutes residence at 270° C. andthat of Polymer E 6 minutes, at a temperature profile from 270°-350° C.

Twelve-inch lengths of each yarn are placed, untensioned, in a mufflefurnace under nitrogen and heated, over the course of 1 hour, to 1100°C. After 30 minutes at that temperature, the furnace is cooled to 200°C. over the course of 3 hours before exposing the carbonized yarns toair. The samples are measured for denier, to determine totalcross-sectional area, and embedded in an epoxy resin.

PREPARATION OF EPOXY COMPOSITE FOR TESTING

1. Weigh 100 parts "Epon" 826 (a product of Shell Chemicals) and 14parts of metaphenylenediamine into a glass container.

2. Dilute with 200 parts acetone. Mix well. This solution must be usedwithin 2 hours after preparation.

3. Pour the solution into a pan of suitable size and into it coil an18-inch (approximately 50 centimeters) or longer strand of a carbon (orgraphite) yarn having a denier of about 1500 (166 tex).

4. Pull the impregnated strand through a glass, fire-polished, eyedropper having a minimum internal diameter of 0.060" (1.5 mm).

5. Clamp the ends of the impregnated, collimated strand between the armsof hinged clamps which have been coated with "Silastic" silicone rubber.The clamps can be made from common 4 inch iron strap hinges by securingtwo bolts to one side of each to permit convenient fastening with nutsafter closing on the impregnated sample.

6. Hang the sample on a horizontal rack by attaching one of the closedclamps to one side of the rack, attaching a 4-pound (1.8 kg) weight tothe second clamp and allowing this weighted end to drape across theother side of the rack, leaving about 15 inches (38.1 cm) of strandsuspended across the opening. Allow the solvent to escape for 2 to 3hours at room temperature.

7. Cure, without removing from the rack, at 120° C. for 2 hours and 155°C. for 4 hours in a circulating air oven.

8. Trim the ends of the strand and measure its length accurately; weightit to the nearest 0.1 milligram. From this weight, the known length andthe known denier of the strand, establish that the resin content of thecomposite is about 40-50% before proceeding further.

9. Sandwich about 2 inches (5 cm) of each end of the strand betweenapproximately 1"×2" (2.5×5 cm) pieces of cardboard along with additionalepoxy resin; clamp the cardboard tabs together and re-cure as in (7)above. This procedure minimizes breakage of the brittle carbon fibers bythe Instron clamp.

10. Test the composite to failure on an Instron in the known manner.

Properties are summarized in the following table.

    ______________________________________                                                      Tensile Strength                                                                            Initial Modulus                                   Composite of  (× 1000 kg/cm.sup.2)                                                                  (× 10.sup.6 kg/cm.sup.2)                    ______________________________________                                        Fiber of Polymer D                                                            Cyclized in air, 96 min.                                                                    10.9          1.1                                               Fiber of Polymer E                                                            Cyclized in air, 6 min.                                                                      9.1          0.8                                               ______________________________________                                    

EXAMPLE 2

Two additional polymers, F and G, are made by the process as generallydescribed above to have intrinsic viscosities of 1.50 and 1.35,respectively. Polymer F consists essentially of 98.8 mol % acrylonitrileand 1.2 mol % sodium styrene sulfonate and serves as a comparison;polymer G consists essentially of 98.8 mol % acrylonitrile and 1.2 mol %ammonium 2-acrylamido-2-methylpropanesulfonate. Both polymers are dryspun into fibers which are drawn to 580% of their as-spun length toyield 700-filament yarns of 1.4 denier/filament.

Stabilization of these yarns is carried out by passing them continuouslythrough a 36", three-zone tube furnace. Each 12" zone has independenttemperature controls. Fibers are processed with equal input and exitspeeds to achieve constant fiber length during stabilization. Hold-uptime is set by selection of yarn speed. All fibers are stabilized underconditions tabulated below in an air atmosphere and have densities aftertreatment of 1.38-1.40 gm/cc.

    ______________________________________                                                  Process Temperature °C.                                                               Hold-Up Time                                         Fiber Composition                                                                         Zone 1  Zone 2   Zone 3                                                                              (Minutes)                                  ______________________________________                                        Polymer F   255     260      270   90                                         Polymer G-Run 1                                                                           250     270      300   60                                         Polymer G-Run 2                                                                           250     270      300   30                                         Polymer G-Run 3                                                                           250     270      300   24                                         Polymer G-Run 4                                                                           250     270      300   17                                         ______________________________________                                    

Increase in Zone #2or Zone #3 by 10° C. beyond those tabulated fortreatment of the fiber of Polymer F cause filament fusion and breaks.Reduction in overall process time from 90 to 60 minutes for treatment ofthe fiber of polymer F results in fiber density below 1.36 gm/cc, theminimum density consistent with good carbonization performance.

After stabilization according to the conditions tabulated, each yarn iscarbonized by passage through a 36° tube furnace heated to 1100° C.which is continuously flushed with nitrogen. Yarn speed is adjusted toprovide 12-minutes exposure. The physical properties tabulated below areobtained after potting the samples at 60% fiber, as described in Example1 (elongation rate--10%/min).

    ______________________________________                                                                     Elongation                                                                            Initial                                  Composition                                                                              Denier  Tenacity  (%)     Modulus                                  ______________________________________                                        Polymer F  515                                                                gpd AVG            5.72      1.83    360                                      kg/cm.sup.2        9.3 × 10.sup.3                                                                    --      0.58 × 10.sup.6                    psi                143 × 10.sup.3                                                                    --      7 × 10.sup.6                       Polymer G-Run 1                                                                          516                                                                gpd avg            7.21      1.58    487                                      kg/cm.sup.2        11.7 × 10.sup.3                                                                   --      0.79 × 10.sup.6                    psi                180 × 10.sup.3                                                                    --      12.2 × 10.sup.6                    Polymer G-Run 2                                                                          579                                                                gpd avg            5.10      1.42    478                                      kg/cm.sup.2        8.3 × 10.sup.3                                                                    --      0.77 × 10.sup.6                    psi                128 × 10.sup.3                                                                    --      12.0 × 10.sup.6                    Polymer G-Run 3                                                                          528                                                                gpd avg            6.78      1.40    526                                      kg/cm.sup.2        11.0 × 10.sup.3                                                                   --      0.85 × 10.sup.6                    psi                169.5 × 10.sup.3                                                                  --      13.1 × 10.sup.6                    Polymer G-Run 4                                                                          509                                                                gpd avg            8.57      1.72    637                                      kg/cm.sup.2        13.9 × 10.sup.3                                                                   --      1.03 × 10.sup.6                    psi                214 × 10.sup.3                                                                    --      15.9 × 10.sup.6                    ______________________________________                                    

These data suggest that improved physical properties are obtained withthe shortest possible stabilization times. Shorter stabilization timesare also desirable for economic reasons.

For a continuous, commercial process, the shortest time of conversioncan be selected for a given precursor fiber, as follows:

1. Determine the stick temperature of the acrylic fibers.

2. Heat samples of the fibers in air at a temperature 10° C. below thestick temperature for increasing periods of time such as 0.25, 0.5, 1.0and 2 minutes using a fresh sample for each test and determine thedensity and stick-temperature for each sample.

3. From the data obtained in 2., select a new temperature 10° C. belowthe stick temperature of a sample for a second short-term treatment ofthat sample at various times at the new temperature.

4. Continuing in this manner, select a temperature for a third and afourth, etc., incremental treatment, always starting the incrementaltreatment at about 10° C. below the then-attained fiber sticktemperature.

5. Plot fiber stick temperature as a function of total time treatment.Select a rate of temperature change from the plotted data which resultsin continuous treatment at a temperature as near as possible to 10° C.below the then-attained stick temperature and program the furnace tooperate at this rate.

If excessive fused filaments are obtained the rate of temperatureincrease should be decreased slightly. By optimization of the rate oftemperature increase, treatments requiring 15-20 minutes as illustratedin the examples can be accomplished in as little as 6 or even 4 minutes.

I claim:
 1. Process for preparing precursor fibers for the preparationof carbon or graphite fibers wherein an acrylic fiber of an acrylicpolymer containing 93-99.4 mol percent acrylonitrile units, 0.6-4.0 molpercent ammonia or amine having a pKb of 5 or less incorporated into thepolymer as neutralizing cations for sulfate or sulfonate end groupsderived from the initiator and activator, if present, and asneutralizing cations for sulfonate groups incorporated into the polymerby copolymerization of one or more copolymerizable, sulfonate containingcomonomers selected from the group consisting of styrene sulfonic acid,allyl sulfonic acid, methallyl sulfonic acid,2-acrylamido-2-methylpropane sulfonic acid and ethylene sulfonic acidand 0-3.0 mol percent of units of one or more neutral comonomersselected from the group consisting of alkyl acrylates having 1-4 carbonatoms in the alkyl group, alkyl methacrylates having 1-4 carbon atoms inthe alkyl group, vinyl acetate, vinyl propionate, styrene, vinylchloride and vinylidene chloride and no more than 0.3 mol percentneutralizing cations other than ammonia or amine having a pKb of 5 orless are heated for 4-20 minutes in one or more stages in air at atemperature at least 10° below the stick temperature of the fibersentering that stage to provide fibers having a density of at least 1.36g/cm³.
 2. Process of claim 1 wherein the fibers are heated in air at250°-360° C.
 3. Process of claim 1 wherein the fibers are heated untilthey have a density of at least 1.40 g/cm³.